London's main airport (8)
I believe the answer is:
heathrow
'london's main airport' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for heathrow that I've seen before include "our counterpart of Charles de Gaulle" , "site for airport" , "London's busiest airport" , "London's main airport" , "lots of movement through the air here" .)